In recent years, the use of sensor networks for monitoring various kinds of information in order to manage for example residential environments, nature conservation, healthcare and traffic conditions has been increasing. In sensor networks, sensors having a communication function (hereafter, referred to as “sensor terminal devices”) are placed at various locations and the sensor terminal devices autonomously form a network. These sensor terminal devices perform wireless communication with sensor terminal devices in the surrounding area by using a random access scheme. Random access communication is a type of communication in which, when data that is to be transmitted is generated, the data is transmitted from the terminal device that generated the data to another terminal device. Examples of random access communication schemes include a carrier sense multiple access/collision avoidance (CSMA/CA) scheme. In this scheme, a sensor terminal device specifies a terminal to transmit data to from among sensor terminal devices in the surrounding area and selects a communication path in accordance with a given communication path selection method. In this way, sensor terminal devices within a sensor network form a wireless multi-hop network. A wireless multi-hop network is a wireless network in which nodes have a function of forwarding data and in which nodes, which are not able to directly communicate with each other, can communicate with each other via nodes interposed therebetween. Each sensor terminal device transmits data to the specified device by using multiple hops. Wireless multi-hop networks are sometimes also called ad hoc networks.
A very large number of wireless terminal devices are used in wireless multi-hop networks such as sensor networks in order to obtain detailed target information. When a network is formed using a very large number of wireless terminal devices, it is desirable that it be possible to purchase the individual wireless terminal devices at a low cost. However, in the manufacture of wireless terminal devices, the individual wireless terminal devices are generally subjected to testing before being shipped out.
In order to reduce costs, wireless terminal devices manufactured at low cost are often only subjected to simple testing. Consequently, there is a risk of there being large manufacturing variations among wireless terminal devices manufactured at low cost. For example, it is thought that reception sensitivity varies greatly among wireless terminal devices manufactured at low cost. Here, reception sensitivity is an index that indicates the minimum reception power at which the reception quality requested for communication can be secured and this minimum reception power is called the “reception sensitivity point”. Wireless terminal devices having a low reception sensitivity point, that is, a high reception sensitivity, are capable of detecting signals at even a small reception power, whereas, it is difficult for wireless terminal devices having a low reception sensitivity to detect signals at a small reception power. Consequently, a situation may occur in which bidirectional communication is difficult between a wireless terminal device in which the reception power is equal to or greater than a reception sensitivity point and another wireless terminal device in which the reception power is less than the reception sensitivity point, despite there being a link therebetween.
In the field of wireless multi-hop networks, the following technologies have been proposed. For example, a related technology has been proposed in which, when a certain terminal is to go offline, the certain terminal searches for a proxy terminal to serve as a substitute for itself and, upon finding such a terminal, the certain terminal instructs the proxy terminal to serve as a substitute for itself. In addition, another related technology has been proposed in which entry permission can be issued to a terminal wishing to newly enter a network by any terminal within an ad hoc network. Yet another related technology has been proposed in which a terminal already placed on a network performs a participation procedure for a terminal that is to be newly placed on the network and assigns a network address to the new terminal.
Japanese Laid-open Patent Publication No. 2003-258811, International Publication Pamphlet No. WO 2008/132808 and Japanese Laid-open Patent Publication No. 2008-206124 disclose examples of the related art.